Low salt levels in the blood is one cause of stomach cramps during running. Other causes of runners cramps are inadequate oxygenation; exposure to large changes in temperature; dehydration.
Many runners experience low Salt levels because of drinking too much water before and during running. The water dilutes the sodium in your body. Sodium is a vital mineral needed for optimal bodily function.
The week before a marathon many runners spend time eating carbohydrates and drinking plenty of water. Few runners step up their salt intake because there is a lot of writing is focused on dangers of eating too much salt.
Salt is as important as water and carbohydrates.